Molecular-Level Insights into Selective Transport of Mg²⁺ in Metal-Organic Frameworks

Metal-organic frameworks (MOF) are promising media for achieving solid-state Mg²⁺ conduction and developing a magnesium-based battery. To this end, the chemical behavior and transport properties of an Mg(TFSI)₂/DME electrolyte system inside Mg-MOF-74 were studied by density functional theory (DFT). We found that inside the MOF chemical environment, solvent and anion molecules occupy the coordinatively unsaturated open metal sites of Mg-MOF-74, while Mg²⁺ ions adsorb directly onto the carboxylate group of the MOF organic linker. These predicted binding geometries were further corroborated by IR spectroscopy. We computed the free energies of desolvation of Mg²⁺ ions inside MOF to investigate the capacity of Mg-MOF-74 thin film to act as a separator for selective Mg²⁺ transport. We showed that Mg-MOF-74 could facilitate partial, but not full, desolvation of Mg²⁺. We found that the dominant minimum-energy pathway (MEP) for Mg²⁺ conduction inside Mg-MOF-74 corresponds to a "solvent hopping" mechanism, with an energy barrier of 4.4 kcal/mol. The molar conductivity of Mg²⁺ associated with the idealized solvent hopping mechanism along the MOF one-dimensional channel was predicted to be 2.4 × 10^-3 S cm^-1 M^-1, which is one to two orders of magnitude greater than the experimentally measured value of 1.2 × 10^-4 S cm^-1 M^-1 (with an estimated Mg²⁺ concentration). We have discussed several possible factors contributing to this apparent discrepancy. The current work demonstrates the validity of the computational strategies applied and the structural models constructed for the understanding of fast and selective Mg²⁺ transport in Mg-MOF-74, which serves as a cornerstone for studying transport of multivalent ions in MOFs. Furthermore, it provides detailed molecular-level insights that are not yet accessible experimentally.